In a typical nuclear reactor, the reactor core includes a number of fuel assemblies, each of which is made up of an array of fuel rods 2, as shown in FIG. 10. Each fuel rod 2 often includes a tubular cladding 4 containing fuel pellets (not shown) composed of fissile material, and the cladding tube is sealed by upper and lower tube end caps or plugs 6. The nuclear reactor core is made up of an array of such fuel rods 2, and is disposed in a pressure vessel containing primary coolant (typically water, although heavy water or another coolant is also contemplated). The primary coolant flows through the nuclear reactor core and is heated by the radioactive core. In a typical boiling water reactor (BWR) configuration, the heated coolant boils to form primary coolant steam that is piped out of the pressure vessel and used to drive one or more turbines. In a typical pressurized water reactor (PWR) configuration, the primary coolant remains in a subcooled state and is piped through steam generators located outside of the vessel to heat secondary coolant that drives one or more turbines. In a variant integral PWR configuration, the steam generators are located inside the pressure vessel and the secondary coolant is pumped into the steam generators.
As noted, each fuel rod 2 typically includes a column of nuclear fuel pellets loaded into a cladding tube 4, and tube end plugs 6 secured to the opposite ends of the cladding tube 4. The end plugs 6 should provide a reliable seal to prevent leakage of primary coolant into the fuel rods 2. In known approaches, the top and bottom end plugs 6 are girth or butt welded to the opposite ends of the tube 4, for example by fusion welding or solid state welding.
Resistance welding of tube end plugs 6 to cladding tubes 4 is also known wherein the cladding tube 4 is butted against the end plug 6 and a controlled high current, typically alternating current (AC), is passed between the cladding tube 4 and the end plug 6 which is compressively loaded. Resistance at the interface between the end plug 6 and the cladding tube 4 generates localized heating resulting in a diffusion bond. While resistance welding has many desirable attributes, the process has some shortcomings. For example, for those nuclear reactors that operate at “high temperatures” (500 C.-° to 950 C.°, the use of special materials, such as martensitic stainless steel, oxide dispersion strengthened alloys (ODS), etc., may be required for construction of the fuel rod's cladding tube and tube end plugs. Although those materials provide enhanced performance characteristics with regard to operation of the fuel rods, they are known to be difficult to weld resulting in undesirable weld properties that may lead to weld failures at an unacceptable rate. Often, because the mass of the tube end plug in the vicinity of the weld region is greater than that of the cladding tube, a large temperature differential between the tube and end plug may lead to degraded bond quality. In order to match the mass of the tube end plug 6 to that of the cladding tube 4 in the vicinity of the weld region, it is known to provide a cylindrical recess 7 in the end of the tube end plug 6 so that the wall thickness are more evenly matched. However, a sharp angular transition 8 from the side wall to the bottom wall of such a recess 7 can lead to stress concentrations and, therefore, reduced strength of the weld region.
Moreover, AC welding has some additional drawbacks. For instance, welding with alternating current causes heating and cooling during each cycle. This can be an important distinction for materials requiring longer welding times (>˜16 ms). When welding with AC, weld time is increased by increasing the number of AC cycles. Within these cycles the material cools, subsequent heating periods within the cycle must reheat the material and overcome the effects of decreased resistance due to deformation at the weld interface (larger surface area).
The present invention recognizes and addresses the foregoing considerations, and others, of prior art constructions and methods.